Glass-ceramics process for their preparation and use

ABSTRACT

The present invention refers to glass-ceramics consisting of the mixtures (I): ZrO 2 —SiO 2 —Me II O, or (II) SiO 2 —Me III O 2 —Me II O, wherein: Me II  is chosen in the group consisting of: Ca, Ba, Mg, Zn or mixture thereof; Me III  is chosen in the group consisting of Al, B or mixtures thereof; each of the above said constituents being present in determined quantities; the invention refers also to a process for preparing the glass-ceramics above defined; porcelain stonewares and glazes containing them and their use for preparing ceramic items.

FIELD OF THE INVENTION

The present invention refers to glass-ceramic consisting of the mixtures (I) or (II):

ZrO₂—SiO₂—Me″O  (I)

SiO₂—Me₂′″O ₃  (II)

wherein in mixture (I):

Me″=Ca, Ba, Mg, Zn or mixtures thereof:

and the percentage, in weight, for each component is:

ZrO₂ 5-25%

SiO₂ 45-75%

MeO″ 15-45%

and in mixture (II)

Me″O is as above defined;

Me′″=Al, B or mixtures thereof

and the percentage in weight for each of the above said components is:

SiO₂ 30-65%

Me₂′″O₃ 5-25%

Me″O 5-40% characterized in that the glass ceramics are obtained in the form of powders.

STATE OF THE ART

It is known that glass is an amorphous material obtained by melting of crystalline compounds followed by cooling down of the melted mass. On the contrary glass-ceramics (hereinafter indicated as GC) are vitreous systems that, when brought to a temperature T₁ higher then their glass transition temperature T_(g), present the formation of crystal nuclei (homogeneous or heterogeneous) with following crystal growth. Porcelain stoneware (also defined as ceramic body having absorption </=0.5%, according to ISO 13006 annnex B1A) is a ceramic material prepared starting from natural crystalline products which, submitted to a syntherization process, partially melt and are transformed into new crystalline phases. This products are prepared starting from a mixture of clay minerals, fondents and possibly eutectic promoters. Glaze is a ceramic product consisting of fondents and silica based glasses which are grinded in granules of the wanted dimensions and thereafter applied on the appropriate substrate and heated so that the grinded granules melt (totally or partially) covering the substrate surface.

All the above said products, having a vitreous surface, confer impermeability and higher physico-chemical properties (better resistance to chemical agents, abrasion etc.) to porous substrates. Moreover they play a very important role as aesthetic materials due to the use of lead based fondents, opacifiers (represented by dispersions of, for example, tin oxide or zirconium silicate which are added in the form of crystals having a determined granulometric dimensions) and coloured pigments.

In JP-A-62 072539 a crystallised glass is described which is obtained by molding glass containing SiO₂ and ZrO₂ as the essential components and crystallising it at 750-1300° C. to deposit tetragonal zirconia in the glass.

However the known materials do not completely satisfy the market needs in so far as the physico-chemical properties are concerned and also their opacization (and the consequent white index) is not always satisfactory giving final products which are aesthetically not suitable for the appropriate use; moreover the known processes for the production of glass-ceramics involve high costs and waste of energy.

DETAILED DESCRIPTION OF THE INVENTION

The invention solves the above said problems thanks to new glass-ceramic having the composition reported above and characterized in that the glass ceramics are obtained in the form of powders.

The glass-ceramic according to the invention can be used as such in order to obtain ceramics or can be added to the materials usually employed for preparing porcelain stoneware or glazes.

The use of the glass-ceramics according to the invention allows (thanks to their “in situ” crystallization) to confer to glaze an exceptional opacization which results in an higher white index (WI) compared to that obtained with the normal opacifiers added to glass, moreover, thanks to the chemical stability and high mechanical resistance of the crystalline phase recrystallized on the glaze surface, they confer to the substrate physico-chemical properties higher then those of the traditional glazes.

The glass-ceramics according to the invention allow also the preparation of ceramics directly from the melted mass which can be shaped in the desired form or the preparation of the wanted ceramic items by tape casting of slurries or hot and cold pressing of the powders.

In the following TABLE 1 preferred glass-ceramic according to the invention are reported (the percentage of the components is given in weight):

TABLE 1 Comp. SiO2 CaO ZrO2 BaO ZnO MgO Al2O3 B2O3 GC.1 55 33 12 GC.2 52.5 31.3 16.2 GC.3 55 21 12 12 GC.4 52.3 21.9 16.4 9.4 GC.5 55 11 12 11 11 GC.6 51.6 39.7 8.7 GC.7 37.8 17.63 37.98 6.59 GC.8 47.95 23.55 9.3 19.2 GC.9 41 0.5 30 20.5 8 GC.10 36.77 47.05 9.75 6.43 GC.11 61 24 15

The glass-ceramics according to the present invention can be prepared according to a process which is substantially similar to the one followed for the production of porcelain stoneware consisting in the grinding/mixing/pressing (and following firing of the pressed materials) of powders with the difference that in this case the powders consists only of glass (i.e. they lack the crystalline starting materials used for the production of porcelain stoneware).

However, it is essential that the firing step is performed according to well defined and controlled thermic cycles in order to develop the wanted properties.

In particular, once the T_(g) (transition temperature) and the T_(c) (crystallization temperature/s) of the material are determined (according to usual methods) the thermic cycle must be performed as follows:

starting from room temperature the powder mixture is heated increasing the temperature by 10°-30° C. per minute up to 350° C. (in order to eliminate the organic impurities according to usual processes) thereafter the heating temperature is increased by 10°-30° C. per minute up to the T_(g) maintaining the temperature at this value for 0-120′, thereafter the temperature is increased by 10°-30° C. per minute up to the T_(c) were it is maintained for 0-several hours, for example up to seven hours, preferably for 0-4 hours, and possibly increased by 10°-30° C. up to the next T_(c) and so on up to the final T_(c); once completed the heating (i.e. once the highest T_(c) has been reached) the mass is cooled down to room temperature.

As above said the glass-ceramics according to the invention can be added to the materials normally used for the production of porcelain stoneware; the adding of the glass-ceramics according to the invention allows the production of ceramics via the process normally used for the production of porcelain stoneware this resulting in an important saving of energy.

For example a glass-ceramic according to the invention is added to a traditional mixture of starting materials for the production of porcelain stoneware; the mixture is loaded in a Alsing ball-mill (continuous or discontinuous) and grinded in the presence of water to give a slurry which is dried in an atomizer forming hollow grains. The grains are shaped in the form of tiles by traditional pressing. The tiles are dried and fired in a traditional furnace at 900°-1230° C. giving the final product. The obtained tiles are constituted of syntherized and crystallized materials and a residual vitreous phase as demonstrated by mineral and X-ray analysis of the file surface.

Preferably the glass-ceramics according to the invention are added to the traditional materials in a quantity comprised between 5% and 65% (in weight). In TABLE 2 it is reported (expressed in metal oxide percentage in weight) an example of the composition of a mixture of starting materials which is added to the glass-ceramics of the present invention in the above reported quantities:

TABLE 2 SiO2 40-80 Al2O3  5-40 MgO 0.1-10  CaO 0.1-10  Na2O 0.1-10  K2O 0.1-10 

In this connection it was surprisingly found, and it is another object of the present invention, that also other glass-ceramics, already known “per se”, can be added to the traditional starting materials for production of porcelain stoneware giving similar advantageous results.

Examples of already known glass-ceramics (a-d) which can be used for the above said purpose are given in following TABLE 3 (the percentage is expressed in weight).

TABLE 3 Glass- ceramic SrO Al2O3 SiO2 ZrO K2O MgO a 30-40 25-30 30-45 b 40-50 10-25 30-4O c 15-25 60-70 10-20 d 20-25 10-25 45-55 5-15

The glazes can be produced via the processes traditionally used for the production of glazes. A glass-ceramic according to the invention and a starting material usually employed for the production of ceramic glazes (which are essentially the same reported for the preparation of porcelain stoneware with the addition of fondents like frits or borates or lead oxide etc.) were loaded in the appropriated proportions in a Alsing ball-mill in the presence of water and grinded to obtain a so called “glaze” which was applied by airbrush, threading die, bell or serigraphy on a traditional substrate, crude or biscuited, obtained by pressing of atomized or dry-grinded powders. The glaze, dried and/or granulated, can be deposited on the substrate surface by falling and fixed on the surface with appropriated ligants. The substrate is fired at 900°-1230° C. in a quick- or tunnel-furnace (continuous or discontinuous) giving the final product in which the glass-ceramic has induced a controlled crystallization. The glazed tiles so obtained present therefore a vitreous and a crystallized part which confer a very well defined microstructure as shown by SEM and X-rays diffractometry.

Preferably the glass-ceramics according to the invention are added to the traditional materials in a quantity comprised between 5% and 60% (in weight).

EXAMPLE 1 Preparation of Porcelain Stoneware (Corresponding to Example 2 in TABLE 4)

GC2 (50% of the total weight) is loaded in a discontinuous Alsing ball-mill together with the traditional starting materials (see ex.2 in TABLE 4) (50% of the total weight).

Water (up to 50% in weight of the material loaded) and 0.4% in weight of sodium tripolyphosphate (as fluidizer) are added.

The mixture is grinded until the slurry residue on a 63 micron sieve is 0.7-1% (in weight).

The slurry is poured into a tank under mechanical stirring and thereafter is spray dried leaving about 6% of water which is the ideal quantity for the following pressing operation.

The humid powder is pressed at 250-500 kg/cm² in the wanted shapes and heated in a furnace at 1230° C.

By repeating the process described in Example 1 but using the products and the quantities indicated in the following TABLES 4-13 other porcelain stonewares were obtained; the glass-ceramics are indicated making reference to TABLE 1 and the quantities are expressed in % in weight:

TABLE 4 Glass-ceramics according to % Glass- % Traditional Traditional Materials invention ceramics materials SiO₂ 76.0 GC2 5 95 Al₂O₃ 17.8 MgO 0.8 CaO 1.0 Na₂O 2.3 K₂O 2.1 SiO₂ 54.6 GC2 50 50 Al₂O₃ 41.8 MgO 0.2 CaO 0.7 Na₂O 0.4 K₂O 2.3 SiO₂ 54.5 GC2 65 35 Al₂O₃ 42.5 MgO 0.2 CaO 0.5 Na₂O 0.3 K₂O 2.0

TABLE 5 Glass-ceramics according to % Glass- % Traditional Traditional Materials invention ceramics materials SiO₂ 3.5 GC3 5 95 Al₂O₃ 18.1 MgO 0.8 CaO 1.0 Na₂O 3.5 K₂O 3.1 SiO₂ 84.0 GC3 50 50 Al₂O₃ 10.3 MgO 0.3 CaO 0.7 Na₂O 2.4 K₂O 2.3 SiO₂ 56.3 GC3 65 35 Al₂O₃ 39.5 MgO 0.3 CaO 0.6 Na₂O 1.3 K₂O 2.0

TABLE 6 Glass-ceramics according to % Glass- % Traditional Traditional Materials invention ceramics materials SiO₂ 74.8 GC4 5 95 Al₂O₃ 20.1 MgO 0.7 CaO 1.1 Na₂O 1.2 K₂O 2.1 SiO₂ 61.3 GC4 50 50 Al₂O₃ 35.3 MgO 0.3 CaO 0.1 Na₂O 1.2 K₂O 1.8 SiO₂ 60.0 GC4 65 35 Al₂O₃ 37.0 MgO 0.2 CaO 0.8 Na₂O 1.0 K₂O 1.0

TABLE 7 Glass-ceramics according to % Glass- % Traditional Traditional Materials invention ceramics materials SiO₂ 72.2 GC5 5 95 Al₂O₃ 15.5 MgO 0.9 CaO 1.3 Na₂O 4.0 K₂O 3.1 SiO₂ 81.5 GC5 50 50 Al₂O₃ 12.0 MgO 0.2 CaO 1.0 Na₂O 2.2 K₂O 3.1 SiO₂ 58.5 GC5 65 35 Al₂O₃ 37.3 MgO 0.3 CaO 0.6 Na₂O 1.0 K₂O 2.3

TABLE 8 Glass-ceramics according to % Glass- % Traditional Traditional Materials invention ceramics materials SiO₂ 76.0 GC9 5 95 Al₂O₃ 17.8 MgO 0.8 CaO 1.0 Na₂O 2.3 K₂O 2.1 SiO₂ 62.8 GC9 50 50 Al₂O₃ 25.8 MgO 0.2 CaO 0.9 Na₂O 7.1 K₂O 3.2 SiO₂ 63.6 GC9 65 35 Al₂O₃ 31.6 MgO 0.3 CaO 1.1 Na₂O 1.0 K₂O 2.4

TABLE 9 Glass-ceramics according to % Glass- % Traditional Traditional Materials invention ceramics materials SiO₂ 75.9 GC6 5 95 Al₂O₃ 17.7 MgO 0.8 CaO 1.0 Na₂O 2.5 K₂O 2.1 SiO₂ 72.8 GC6 50 50 Al₂O₃ 10.1 MgO 6.7 CaO 0.2 Na₂O 7.5 K₂O 2.7 SiO₂ 67.0 GC6 65 35 Al₂O₃ 20.0 MgO 2.2 CaO 0.4 Na₂O 7.8 K₂O 2.6

TABLE 10 Glass-ceramics according to % Glass- % Traditional Traditional Materials invention ceramics materials SiO₂ 74.0 GC7 5 95 Al₂O₃ 14.2 MgO 1.5 CaO 1.5 Na₂O 4.2 K₂O 4.6 SiO₂ 72.6 GC7 50 50 Al₂O₃ 14.8 MgO 1.3 CaO 2.0 Na₂O 9.0 K₂O 0.3 SiO₂ 65.9 GC7 65 35 Al₂O₃ 23.6 MgO 0.2 CaO 0.9 Na₂O 9.1 K₂O 0.3

TABLE 11 Glass-ceramics according to % Glass- % Traditional Traditional Materials invention ceramics materials SiO₂ 74.3 GC8 5 95 Al₂O₃ 15.8 MgO 1.5 CaO 2.2 Na₂O 3.7 K₂O 2.5 SiO₂ 68.0 GC8 50 50 Al₂O₃ 26.8 MgO 2.2 CaO 0.4 Na₂O 1.0 K₂O 1.6 SiO₂ 67.0 GC8 65 35 Al₂O₃ 28.0 MgO 1.8 CaO 1.2 Na₂O 0.3 K₂O 1.7

TABLE 12 Glass-ceramics according to % Glass- % Traditional Traditional Materials invention ceramics materials SiO₂ 74.0 GC10  5 95 Al₂O₃ 17.6 MgO 1.8 CaO 1.4 Na₂O 3.7 K₂O 1.5 SiO₂ 69.0 GC10 50 50 Al₂O₃ 26.8 MgO 1.2 CaO 0.4 Na₂O 1.0 K₂O 1.6 SiO₂ 67.9 GC10 65 35 Al₂O₃ 28.5 MgO 1.0 CaO 1.1 Na₂O 0.4 K₂O 1.1

TABLE 13 Glass-ceramics according to % Glass- % Traditional Traditional Materials invention ceramics materials SiO₂ 78.0 GC11  5 95 Al₂O₃ 10.8 MgO 2.2 CaO 0.4 Na₂O 6.0 K₂O 2.6 SiO₂ 70.1 GC11 50 50 Al₂O₃ 14.9 MgO 8.7 CaO 0.1 Na₂O 4.3 K₂O 1.9 SiO₂ 69.0 GC11 65 35 Al₂O₃ 27.0 MgO 1.0 CaO 1.1 Na₂O 1.0 K₂O 0.9

EXAMPLE 2 Preparation of Glazes (Corresponding to Example 1 in TABLE 14)

GC2 (30% in weight) was loaded in a discontinuous Alsing ball-mill together with the traditional starting materials (see Ex.1 in TABLE 14) (50% of the total weight).

Water (up to 50% in weight of the total material loaded), sodium tripolyphosphate (as fluidizer) (0.4% in weight) and hydroxymethylcellulose (0.3%) (as ligant) are added.

The mixture is grinded until the slurry residue on a 16000 micron sieve is about 2%.(in weight).

The slurry is bell applied (500 g-2.5 Kg) on a pressed substrate which is heated in a furnace at 1160° C.

By repeating the process described in Example 2 but using the products indicated in the following TABLES 14-23 other glazes were prepared; the glass-ceramics are indicated making reference to TABLE 1 and the quantities are expressed in % in weight.

In each TABLE are reported the starting materials and the corresponding to quantities for the preparation of two different glazes (indicated as Glaze I and Glaze II) starting from the same glass-ceramic according to the invention and the same traditional starting compounds but using different quantities of the same.

TABLE 14 Traditional starting materials and/or frits Glass-ceramic Glaze I Glaze II GC2 30 50 SiO₂ 54.0 70 50 Al₂O₃ 33.0 MgO 0.3 CaO 0.7 Na₂O 8.0 K₂O 4.0

TABLE 15 Traditional starting materials and/or frits Glass-ceramic Glaze I Glaze II GC3 30 50 SiO₂ 60.0 70 50 Al₂O₃ 31.8 MgO 0.2 CaO 0.8 Na₂O 6.2 K₂O 1.0

TABLE 16 Traditional starting materials and/or frits Glass-ceramic Glaze I Glaze II GC4 30 50 SiO₂ 60.0 70 50 Al₂O₃ 31.2 MgO 0.4 CaO 0.8 Na₂O 6.0 K₂O 1.6

TABLE 17 Traditional starting materials e/o frits Glass-ceramic Glaze I Glaze II GC5 30 50 SiO₂ 60.0 70 50 Al₂O₃ 28.0 MgO 0.0 CaO 5.0 Na₂O 3.0 K₂O 2.0 SnO₂ 2.0

TABLE 18 Traditional starting materials and/or frits Glass-ceramic Glaze I Glaze II GC9 30 50 SiO₂ 58.0 70 50 Al₂O₃ 18.0 MgO 2.5 CaO 10.5 Na₂O 6.0 K₂O 5.0

TABLE 19 Traditional starting materials and/or frits Glass-ceramic Glaze I Glaze II GC6 30 50 SiO₂ 59.0 70 50 Al₂O₃ 10.0 MgO 1.0 CaO 7.0 Na₂O 8.0 K₂O 3.0 ZrO₂ 12.0

TABLE 20 Traditional starting materials and/or frits Glass-ceramic Glaze I Glaze II GC7 30 50 SiO₂ 60.0 70 50 Al₂O₃ 17.1 MgO 2.4 Na₂O 6.2 K₂O 4.3 ZrO₂ 8.0 TiO₂ 2.0

TABLE 21 Traditional starting materials and/or frits Glass-ceramic Glaze I Glaze II GC8 30 50 SiO₂ 54.0 70 50 Al₂O₃ 26.0 MgO 0.1 CaO 8.5 Na₂O 11.0 K₂O 0.2 TiO₂ 0.2

TABLE 22 Traditional starting materials and/or frits Glass-ceramic Glaze I Glaze II GC10 30 50 SiO₂ 62.0 70 50 Al₂O₃ 6.3 MgO 1.3 CaO 14.0 Na₂O 0.5 K₂O 5.6 ZnO 10.3

TABLE 23 Traditional starting materials and/or frits Glass-ceramic Glaze I Glaze II GC11 30 50 SiO₂ 50.0 Al₂O₃ 25.0 70 50 MgO 10.0 Na₂O 7.0 K₂O 6.0 TiO₂ 2.0

EXAMPLE 3 Preparation of Glass-ceramic

In this case the powder is prepared according to Example 1 but only GC2 is used. The tile according to Example 1 is prepared applying the thermic cycle C1 reported in TABLE 24 wherein the T_(g) and the T_(c1)and T_(c2) of the starting materials (GC2) are also indicated.

In TABLE 25 thermic cycles for the preparation of compound GC8 are reported; the T_(g) and T_(c) of GC8 are also indicated.

All the Glass-ceramics obtained via the described processes showed a typical superficial micro-structure presenting several crystalline and vitreous phases.

TABLE 24 GC 2 Starting product T_(g) = 825° C. T_(c1) = 966° C. T_(c2) = 1025° C. Thermic cycles applied: B1 C1 C2 total Total Total Step Time T Step Time 1 Step Time min (min) (° C.) min (min) T (° C.) min (min) T (° C.)  0  0  25  0  0  25  0  0  25 35  35 350 35  35 350 35  35 350 30  65 350 30  65 350 30  65 350 55 120 900 55 120 900 55 120 900 30 150 900 30 150 900 30 150 900  8 158 980  8 158 980 25 175 1150  30 188 980 30 188 980 30 205 1150  17 205 1150  22 210 1200  30 205 1150  30 235 1150  30 240 1200 

TABLE 25 GC8 starting material T_(g): 740° C. T_(c) 934° C. Thermic cycles applied: B1 C1 C2 Total Total Total Step Time T Step Time Step Time min (min) (° C.) min (min) T (° C.) min (min) T (° C.)  0  0  25  0  0  25  0  0  25 35  35 350 35  35 350 35  35 350 30  65 350 30  65 350 30  65 350 44 109 790 44 109 790 44 109 790 30 139 790 30 139 790 30 139 790 16 155 950 26 165 1050  36 175 1150  30 185 950 30 195 1050  30 205 1150  

What is claimed is:
 1. A glass-ceramic in the form of a powder which has the formula: SiO₂—Me₂′″O₃—Me″O wherein Me″ is selected from the group consisting of Ca, Ba, Mg, Zn and mixtures thereof; wherein Me′″ is selected from the group consisting of Al, B and mixtures thereof and the weight percent of each of SiO₂; Me₂′″O₃; and Me″O is: SiO₂=30-65%; Me₂′″O₃=5-less than 20%; and Me″O=5-40%.
 2. A glass-ceramic as described in claim 1 having the following composition in weight percent which is selected from the group consisting of: SiO₂—ZnO—Al₂O₃ (51.6:39.7:8.7) SiO₂—CaO—BaO—Al₂O₃ (37.8:17.63:37.98:6.59); SiO₂—CaO—MgO—Al₂O₃ (47.95:23.55:9.3:19.2); SiO₂—BaO—MgO—Al₂O₃ (36.77:47.05:9.75:6.43); and SiO₂—CaO—Al₂O₃ (61:24:15).
 3. Process for the preparation of glass ceramics from a powder mixture which has the formula: SiO₂—Me₂′″O₃—Me″O wherein Me″ is selected from the group consisting of Ca, Ba, Mg, Zn and mixtures thereof; wherein Me′″ is selected from the group consisting of Al, B and mixtures thereof and the weight percent of each of SiO₂; Me₂′″O₃; and Me″O is: SiO₂=30-65%; Me′″O₃=5-less than 20%; and Me″O=5-40% wherein: the starting materials are submitted to grinding, mixing and pressing steps for the production of porcelain stoneware to form a pressed powder mixture, and then firing said pressed powder mixture as follows: starting from room temperature said pressed powder mixture is heated by increasing the temperature from room temperature at the rate of 10°-30° C. per minute up to 350° C., to eliminate organic impurities, and thereafter increasing the temperature at a rate of 10°-30° C. per minute up to the T_(g); maintaining the T_(g) temperature for 0-120 minutes and thereafter increasing the temperature at a rate of 10°-30° C. per minute up to the T_(c) where it is maintained for 0-several hours and optionally increasing the temperature at a rate of 10°-30° C. per minute up to the next T_(c) and up to the final T_(c); and after the highest T_(c) has been reached, cooling the glass ceramic down to room temperature.
 4. Porcelain Stoneware consisting of a glass-ceramic according to claim 1, wherein the mixture containing a traditional mixture of metal oxides SiO₂ (40-80), Al₂O₃ (5-40), MgO (0.1-10), CaO (0.1-10), Na₂O (0.1-10), and K₂O (0.1-10) in which said traditional metal oxides are expressed in percent by weight, mixed with a glass-ceramic selected from the group consisting of: SiO₂—ZnO—Al₂O₃ (51.6:39.7:8.7); SiO₂—CaO—BaO—Al₂O₃ (37.8:17.63:37.98:6.59); SiO₂—CaO—MgO—Al₂O₃ (47.95:23.55:9.3:19.2); SiO₂—BaO—MgO—Al₂O₃ (36.77:47.05:9.75:6.43); and SiO₂—CaO—Al₂O₃ (61:24:15); represents 5%-65% by weight of the total weight of said Porcelain Stoneware.
 5. Porcelain Stoneware consisting of a crystalline powder having a chemical formula as recited in claim 2, in admixture with a mixture of metal oxides comprising SiO₂ (40-80 wt. %), Al₂O₃ (5-40 wt. %), MgO (0.1-10 wt. %), CaO (0.1-10 wt. %), Na₂O (0.1-10wt. %), and K₂O (0.1-10 wt. %), whereby said crystalline powder represents 5% to 60% by weight of said Porcelain Stoneware.
 6. Ceramic glaze consisting of a glass-ceramic SiO₂ ZnO Al₂O₃ according to claim 2, and metal oxides of the formula: SiO₂ 59.0% by weight; Al₂O₃ 10.0% by weight; MgO 1.0% by weight; CaO 7.0% by weight; Na₂O 8.0% by weight; K₂O 3.0% by weight; and ZrO₂ 12.0% by weight; the weight being based on the total weight of all the components; wherein the weight ratio of SiO₂ ZnO Al₂O₃ to the metal oxides is 30 to
 70. 7. Ceramic glaze consisting of a glass-ceramic SiO₂ CaO BaO Al₂O₃ according to claim 2 which is mixed with a traditional metal oxide mixture consisting essentially of SiO₂ 60.0 wt. %; Al₂O₃ 17.1 wt. %; MgO 2.4 wt. %; Na₂O 6.2 wt. %; K₂O 4.3 wt. %; ZrO₂ 8.0 wt. %; TiO₂ 2.0 wt. %; in a ratio by weight of 30 to 70 of the glass-ceramic to said traditional metal oxide mixture.
 8. Ceramic glaze consisting of a glass-ceramic SiO₂ CaO MgO Al₂O₃ according to claim 2 which is mixed with a metal oxide mixture consisting essentially of SiO₂ 54.0 wt. %; Al₂O₃ 26.0 wt. %; MgO 0.1 wt. %; CaO 8.5 wt. %; Na₂O 11.0 wt. %; K₂O 0.2 wt. %; TiO₂ 0.2 wt. % in a ratio by weight of 30 to 70 of the glass-ceramic to said metal oxide mixture.
 9. Ceramic glaze consisting of a glass-ceramic SiO₂ CaO ZnO Al₂O₃ according to claim 2 which is mixed with a metal oxide mixture consisting essentially of SiO₂ 58.0 wt. %; Al₂O₃ 18.0 wt. %; MgO 2.5 wt. %; CaO 10.5 wt. %; Na₂O 6.0 wt. %; K₂O 5.0 wt. %; in a ratio by weight of 30 to 70 of the glass-ceramic to said metal oxide mixture.
 10. Ceramic glaze consisting of a glass-ceramic SiO₂ BaO MgO Al₂O₃ according to claim 2 which is mixed with a metal oxide mixture consisting essentially of SiO₂ 62.0 wt. %; Al₂O₃ 6.3 wt. %; MgO 1.3 wt. %; CaO 14.0 wt. %; Na₂O 0.5 wt. %; K₂O 5.6 wt. %; ZnO 10.3 wt. % in a ratio by weight of 30 to 70 of the glass-ceramic to said metal oxide mixture.
 11. Ceramic glaze consisting of a glass-ceramic SiO₂ CaO Al₂O₃ according to claim 2 which is mixed with a metal oxide mixture consisting essentially of SiO₂ 50.0 wt. %; Al₂O₃ 25.0 wt. %; MgO 10.0 wt. %; Na₂O 7.0 wt. %; K₂O 6.0 wt. %; TiO₂ 2.0 wt. % in a ratio by weight of 30 to 70 of the glass-ceramic to said metal oxide mixture. 